The present invention relates to an adaptive process for the coding and decoding of a sequence of images or pictures by transformation and to a device for performing this process. The object of such a process is to reduce the quantity of information to be transmitted or stored, when video pictures have been digitized.
It is known to effect a coding and decoding of a video picture by a bidirectional transformation of the cosine, Fourier, Hadamard, Haar, or Karhunen Loeve. Coding consists of dividing each picture into square blocks, applying the transformation to each block to obtain a matrix of values called transformation coefficients of the block and then transmitting these coefficients in a coded form. Decoding consists of decoding the coefficients and then restoring the digital values representing each point of a block by applying the inverse transformation to the matrix of the transformation coefficients of said block. The transformations used in practice are those for which there are fast algorithms, e.g. Fourier, Hadamard, Haar, and cosine transformations.
The IEEE article "Transactions on Communications", vol COM, 32, No 3, March 1984 describes an adaptive coding process consisting of quantifying the transformation coefficients in an adaptive manner, so that the blocks of coefficients containing a large amount of energy are quantizied with a larger number of quantification levels and a larger number of code bits than the blocks containing little energy. The image or picture is divided into 16.times.16 blocks. The digital values representing the points of each block are transformed by cosine transformation and then the thus obtained transformation coefficients are compared with a threshold. The coefficients higher than this threshold are normalized by multiplying them by a parameter supplied by a device for regulating the transmission rate.
The thus normalized coefficients are then quantizied and coded by a Huffman code, followed by storage in buffer memory which regulates the transmission rate. This buffer memory stores at a variable clock cycle the information to be transmitted, said clock cycle being dependent on the instantaneous energy of the picture. The buffer memory transmits the information at a constant rate. The degree of filling of this buffer memory and the rate of its input are monitored, and this determines the value of the normalized parameter.
Following transmission, the coded information arrives at a constant rate into a buffer memory, which passes them at a variable rate to a Huffman decoder. The transformations are then normalized by a normalization parameter which is the inverse of that used for coding, said inverse parameter being calculated by a rate regulating device which monitors the degree of filling of the buffer memory. The transformation coefficients are then added to the threshold value used for coding and are transformed by inverse transformation of the cosine transformation to restore the digital values representing points of a block.
The quality of the pictures restored is very good compared with other known methods, provided that the pictures correspond to highly animated (quickly changing) scenes. The visual quantity of the restored pictures is far less acceptable when they show only slightly animated or completely fixed scenes. In the latter case, two faults appear. It is possible to see a filtering of the high and medium spatial frequencies, said filtering corresponding to the elimination of low energy coefficients, which have a value below the threshold. It is also possible to see demarcations between the blocks in the uniform zones of the pictures, because the eye is sensitive to limited brightness differences in the uniform zones.
The object of the present invention is a coding and decoding process not suffering from the above disadvantage sense it permits an excellent restoration of fine details when the pictures show slightly animated or completely fixed scenes. The invention takes advantage of the redundancy of information existing in the sequence of pictures.